The sleep-wake rhythm of humans generally consists of the wakefulness during day-time and the sleep during night-time. The time in sleep is not a homogenous period of time but consists of different phases of sleep called “sleep stages” including Stage 1 to Stage 4 (S1-S4) and REM (rapid eye movements) stages, which can be detected and characterized using the electrophysiological measurements called polysomnography. A typical feature of different sleep stages is that the stimuli to wake up the sleeping subject are different. During the wakefulness the stimuli are followed actively by the subject. In drowsiness and very light sleep (Stage 1) the stimulus to awake is small and in the deep sleep (“slow wave sleep”, i.e. Stage 2 and especially Stage 3-4 and REM sleep) the stimulus required is strong and the wakening up the person and the awakening experienced by the sleeping subject are difficult and the awakening is experienced troublesome and unpleasant. The sleep stages succeed each other after the onset of the sleep. First the sleep becomes deeper and thereafter the deepness of the sleep varies in repeated periods (sleep cycles) of about 90 minutes. Between the sleep cycles the sleeping subject often changes his/her body position and then the sleep is especially light during the body movements and the sleeping subject is actually awake during the short periods of movements to refall asleep immediately. The sleep cycle is clear and regular during the first, evening hours of the sleep but it becomes more irregular during the dawn (last) hours of the sleep and the body movements, changes in body position, and changes of the sleep stages into deeper and lighter sleep are more frequent during the last hours of the night sleep. It is typical in many sleep disorders, including insomnia, that the cycling pattern of the sleep constructed by the different sleep stages becomes irregular and poor.
During the deep sleep (“slow wave sleep” in electrophysiological measurements like PSG) the muscle tone of the sleeping subject is very low, the breathing (respiration) is peaceful and there are no special movements of the body or the limbs present. During the REM sleep the muscle tone is very low but very short movements like twitches of muscles may still take place. The typical feature of REM sleep is the rapid eye movements setting the name to the whole sleep stage. REM sleep is present predominantly during the last hours of sleep. During REM sleep and these short twitches of the muscles the wakening and awakening is not easy. The duration of the changes in the body and limb positions is several seconds. These movements are possible only using the big body muscles so that the sleeping subject in the electrophysiological measurement is actually detected to be awake for a short period of time although he/she cannot remember it afterwards.
If the duration of the night sleep is sufficiently long, the awakening may happen spontaneously and without any troubles in the morning when there is no more pressure to sleep and the sleeping subject feels vigilant and alert. If the sleep-wake cycle is regular and the amount of the sleep is sufficient, the human body with the internal rhythms of the brain and the other organs may even learn to awake spontaneously regularly without any alarm clock. The schedules may not always be regular, however. Because the social schedules also are important and the sense of elapsed time is not present during the sleep, an alarm clock is often required to wake up the sleeping subject at a specified, fixed time of day. It the alarm clock is alarming during the deep slow wave sleep or during REM sleep the awakening may be especially troublesome, slow and even an unpleasant experience. If the awakening occurs at a moment when the sleeping subject is in light sleep (in drowsiness) or e.g. changing the sleeping position (arousal) and therefore actually awake, the awakening takes place easily without troubles and even due to a minimal stimulus, which also may be a natural stimulus like voice, light, or other similar stimulus from the surrounding. This kind of awakening during the drowsiness (light sleep) or during spontaneous arousal or the wake state is thus obviously advantageous to the human organism and well-being. The wake-up due to the alarm clock at an accurately fixed time point may obviously happen either advantageously during light sleep stage or unfavourably during the deep sleep.
When the quality of sleep of a sleeping person, especially of a baby, is monitored, the parents of the baby or the other persons wish to monitor and follow up the well being, calmness and awakenings of the sleeping child. The voices due to crying may be monitored from distances using a wireless radio frequency transmitter (“cry detector”) located near to the baby which allow the parents in another room to notice that their baby is crying and not sleeping. In that case they may come to look for the status of the baby and, if required, peacefully pacify the baby to induce sleep again. These alarming devices are thus used to detect that the sleeping subject has awakened. The detection of the awakening of the baby is based on the detection of signals which are higher than the triggering level which has been set as the criterion for the alarm.
One prior art solution to wake up a person is called “Sleep Tracker” (www.sleeptracker.com) based on the publication “Easy Wake Wrist Watch” US 2005/0154330. This device is a “wrist watch” combined with a movement sensor (sensor for acceleration, “actigraph”). The device is monitoring the movements of the body using the signals from the acceleration sensors throughout the night and it aims to notice different kinds of sleep stages (stage with no movements as the deep sleep and movements during transitions between the sleep stages). The user of the device determines a time window which is the time period during which he wants to wake up. If the device detects, using the signal from the movement sensor, an advantageous type of epoch of a sleep cycle, the alarm clock of the wrist watch performs the wake up. Same type of methods and devices to detect arousal state has been based on the analysis of brain electric activity (electroencephalography) and eye movement electric activity (electro-oculography).
The aforementioned devices have to be purchased separately (the sensor and alarming device fixed to the wrist, separate sensor and alarming devices, amplifiers, measuring devices of EEG or EOG signals). The present methods also often require the sensor devices to be fixed permanently to the sleeper causing discomfort and making sleeping troublesome. In addition, if utilized outside permanent home, the devices have to be carried along and be prepared for use. Accordingly, prior art solutions are often prone to various environmental conditions that may deteriorate the performance of used measurement and analysis methods in real-life scenarios wherein the prevailing conditions often differ from time to time.